The present invention relates to a technique of manufacturing a semiconductor integrated circuit device, and particularly to a technique advantageously applied to cleaning process for a silicon wafer.
In the process of manufacturing a LSI using a wafer made of mono-crystalline silicon, a wafer cleaning technique (so-called RCA cleaning) proposed by RCA (a company name in the U.S.A.) has been widely used (ref. W. Kern et al., RCA Review, 31 (1970), p. 187).
The RCA cleaning is a cleaning technique based on a combination of (1) dipping process at 80.degree. C. for about 10 to 20 minutes using a mixed solution (SC-1 solution or APM solution) of ammonia:hydrogen peroxide:water=1:1:5 (in volume ratio), (2) dipping process at a room temperature for several ten seconds using a mixed solution (DHF solution) of hydrofluoric acid:water=1:99, and (3) dipping process at 80.degree. C. for about 10 minutes using a mixed solution (SC-2 solution or HPM solution) of hydrochloric acid:hydrogen peroxide:water=1:1:5. The SC-1 solution is used mainly to remove fine particles and organic contamination. The DHF solution is used mainly to remove a silicon oxide film, and the HPM solution is used mainly to remove metal contamination.
In recent years, proposals have been made of various cleaning techniques achieved by improving the RCA cleaning.
Japanese Patent Application Laid-Open No. 5-136119 discloses a technique in which phosphoric acid or phosphate is added to the APM solution thereby to restrict autolysis of hydrogen peroxide so that time-based changes such as an etching speed and the like are reduced.
Japanese Patent Application Laid-Open No. 61-60799 discloses a technique in which ammonium salt (for example, ammonium phosphate) is added to the APM solution thereby to restrict corrosion of Al (aluminum) wires.
Japanese Patent Application Laid-Open No. 6-216098 discloses a technique by which the contamination removal effect of the APM solution is raised and the RCA cleaning process is simplified. The technique is that cleaning is carried out with use of a mixed solution of the APM solution and complexone (such as EDTA or the like) of 0.1 to 100 ppm added thereto or a chelating agent obtained by substituting the ligand of the carboxylic acid radical of the complexone with another acid radical, and then rinsing is carried out using water added with hydrofluoric acid of 1 ppm or more.
Japanese Patent Application Laid-Open No. 6-41773 discloses a technique in which an anti-adsorption agent (such as NTPO, EDTPO, EDDPO, or the like) is added to an aqueous solution containing an alkaline component such as ammonium hydroxide, hydroxide of alkaline metal (KOH or NaOH), choline, or the like, thereby to restrict adsorption of metal into the wafer surface.
Japanese Patent Application Laid-Open No. 6-177101 discloses a technique in which a nonionic surface active agent (such as polyoxyethylene nonyl ether) is added to the APM solution thereby to form a foam layer on the solution surface so that scattering of ammonia is prevented.
Japanese Patent Application Laid-Open No. 7-245281 discloses a cleaning solution consisting of an alkaline oxidative aqueous solution which contains hydroxide of alkaline metal (KOH or NaOH) of 0.05 mol/l to 5 mol/l and hydrogen peroxide of 0.02 mol/l to 0.5 mol/l (and which may further contain a surface active agent such as alkyl benzene sulfate salt). In this cleaning solution, the pH of the solution is raised for adding the hydroxide of alkaline metal as a strong alkaline material, so that detergency with respect to strong contamination which are difficult to remove by normal RCA cleaning. Also, in this cleaning solution, the concentration of the hydroxide of alkaline metal and the concentration of the hydrogen peroxide are limited within such ranges that reaction proceeds uniformly at a relatively slow speed, and that unevenness of etching is not caused due to increase of the reaction speed.
In case of the APM solution used in the RCA cleaning, foreign materials are removed by etching a silicon oxide film by a hydroxyl group (OH.sup.-). However, a silicon wafer is slightly etched by the hydroxyl group. In addition, in the APM solution, the hydroxyl group has a high concentration since ammonia has a high mixing ratio.
Therefore, in the cleaning process having used the APM solution, because being etched, the surface of the wafer is roughened and the flatness thereof is deteriorated. The deterioration of the flat surface affects adversely a boundary characteristics of the interface between the silicon wafer and a gate oxide film formed on the surface of the silicon wafer and causes deterioration of device characteristics such as reductions of the amount of the drive current and the mobility of electrons. This is a serious problem particularly for the process of forming a gate of a MOSFET which requires such a thin gate oxide film of high quality as to have a film thickness of several nm.
Also, the cleaning process using the APM solution requires processing at a high temperature (80.degree. C.) for a long time (10 to 20 minutes) because the etching speed of the silicon oxide film based on a hydroxyl group is low. However, as a sheet-by-sheet processing in accordance with increase of the diameter of the wafer progresses, brief processing time of the cleaning process is required from the viewpoint of improving the throughput. Further, there is a problem that the operation cost for the cleaning device increases for performing the cleaning process at a high temperature.